Acid / Base Equilibria
Lowry-Bronsted Definitions
- acid
- a proton donor
- base
- a proton acceptor
Protonated vs Deprotonated states
I'm a protonated state.
Why? because I HAVE the proton. You might know me best as a plain acid, HA. But remember, I can also be a protonated base, BH+. It doesn't matter which version I am, I still behave as an acid and I donate this proton to a base. My acidic strength is measured by my value for Ka.
I'm a deprotonated state.
Why? because I DON'T HAVE the proton. You probably know me best as a plain base, B. I also exist as a deprotonated acid, A-. I will always behave as a base because I am ready to accept a proton from an acid. My base strength is measured by my value for Kb.
If the only difference between me and my buddy there is that one single proton, then we are a conjugate acid/base pair and have a special relationship which is that: Ka· Kb = Kw
Question:
Why did Dr. McCord pick the color scheme that he did for the pH scale? You should know.
Four Problems Solved the same way...
SO MANY Problems in acid/base theory are answered via the following generic set up.
\[K = {x^2\over C - x}\]
K will either be Ka or Kb and you will typically solve for x and it will be either [H+] (acid problems) or [OH-] (base problems). You then take the log of x and you'll have either pH or pOH.
IF K is small enough (say < 10-4), then the following approximation is valid:
\[K \approx {x^2\over C}\]
and easily solved without using the quadratic formula...
\[x = \sqrt{K\cdot C}\]
Example wordings of questions that ultimately solve like shown above
- What is the pH of a 0.??? M solution of ?????
- What is the percent ionization of ???? in a solution of 0.??? M ?????
- What percentage is protonated in...
- What percentage is deprotonated in...
- Given that a solution of 0.??? M of ???? has a pH of ??.??, what is the value of K?